Normally gaseous materials are cooled for a variety of purposes. One major area involves the cooling of a normally gaseous material to separate all or some of the components of the normally gaseous material. For example, air is separated into its constituents by compressing the air to a pressure substantially above ambient pressure and, thereafter, passing the same through a series of cooling cycles. The components of the air are successively condensed and separated, the lowest boiling component being separated first, thereafter, intermediate boiling components and, finally, the highest boiling component. The cooling media may be one or more external refrigerants or a portion or portions of the cold gases or liquids produced in the process. Usually, the cooling medium is a combination of external refrigerants and cold gases or liquids produced in the process, since the use of the latter eliminates the need for additional external coolants as well as recovers some of the energy utilized in the system. Another process in which cooling of the normally gaseous material is utilized to separate components of the gas is in the processing of natural gas. While natural gas predominates in methane, such gases often contain contaminants such as nitrogen, as well as materials which are more valuable for other uses than the methane, which is most often utilized as a domestic or industrial fuel. Consequently, these contaminants and/or more valuable components are removed from the natural gas by the same type of cooling utilized in the separation of air into its components. For example, contaminants such as nitrogen, which can be included in a lean fuel utilized for in-plant purposes, or recovered as a product, helium, which may be recovered as a product, C.sub.2, C.sub.3, and C.sub.4 hydrocarbons, which are valuable chemical feedstocks, and normally liquid C.sub.5 and higher molecular weight hydrocarbons, which are valuable gasoline blending stocks as well as chemical feedstocks, are generally separated from the natural gas.
Another major reason for cooling normally gaseous materials is to liquify the gas or portions thereof for purposes of storage and transportation. Except for the fact that the gas must be cooled to a lower temperature to liquify the same, as opposed to separating the components thereof, the basic techniques utilized in liquifaction of a normally gaseous material are the same as those utilized in the separation of components of a normally gaseous material.
Cooling for the purposes of liquifying a normally gaseous material is also practiced in the processing of natural gas.
The processing of natural gas is illustrative of both the separation of components from a normally gaseous material, as well as the liquifaction of a portion thereof. A highly effective process of this type is illustrated and described in U.S. Pat. No. 4,430,103, which is incorporated herein by reference. While natural gas is usually at a pressure above ambient pressure as produced from an underground formation, in order to separate the components thereof and/or liquify the same it is generally necessary to compress the gas to a still higher pressure. For example the pressure of the feed gas to a natural gas processing system may vary anywhere between 100 and 5000 psia, the pressure will usually be between about 300 and 1500 psia, and in most instances between about 500 and 900 psia. In addition, it is necessary to compress the refrigerants utilized in the process. Obviously, such compression of the feed and the refrigerants requires substantial amounts of energy and it would be highly desirable to reduce these horsepower requirements of the system. As illustrated by FIG. 1 of the above-mentioned patent, the feed gas stream, at a pressure substantially above ambient pressure, is sequentially passed through three cooling cycles, usually comprising a propane cycle, an ethylene or ethane cycle and a methane cycle, and is, thereafter, reduced in pressure to ambient pressure for storage or transportation. During the course of cooling in the propane and ethane or ethylene cycles, C.sub.2 and higher molecular weight hydrocarbons sequentially condense in accordance with their respective condensation temperatures. These components are removed, to the extent possible, by withdrawing the feed gas stream from the cooling sequence at appropriate temperatures, separating the liquid phase from the gas phase and returning the gas phase to the cooling sequence. The recovered liquid phase materials may then be separated into constituent streams by, for example, fractionation, as is also shown in FIG. 1 of the patent drawing. The feed gas stream will generally be in a liquid phase at a pressure somewhat below the original pressure but still substantially above ambient pressure as it leaves the ethane or ethylene cooling cycle. In order to reduce the pressure to ambient pressure for storage and/or transportation, the pressure is then reduced in an expansion cycle. Conveniently, the cold vapors flashed from the main gas stream in the pressure reduction cycle provide the refrigerant for the methane cooling cycle. Nitrogen is removed from the feed gas stream during the methane cooling cycle by withdrawing the feedgas stream from the cooling sequence and fractionating the stream, to produce a vapor phase enriched in nitrogen, or by a plurality of expansion stages or a combination of both, as shown in FIG. 1 of the patent. The cold vapors produced in the expansion cycle are utilized in the methane cooling cycle, as previously indicated, and are then compressed, cooled and returned to the main gas stream. While the temperature of the recycled methane will generally be approximately the same as the temperature of the main gas stream at the point which it is recombined, the pressure of the recycled methane will generally be below the pressure of the main gas stream. As a result, in order to compensate for this reduction of pressure, caused by recycling of the lower pressure methane, it is necessary that the initial pressure of the feed gas stream be higher. It would therefore be highly desirable to eliminate this reduction of pressure of the main gas stream at this point and thereby reduce the horsepower requirement necessary for initially compressing the feed gas stream.
It is also obvious, from FIG. 1 of the drawings of the patent, that each of the three cooling cycles includes a plurality of separate coolings stages, namely, three stages in the propane cycle, four stages in the ethane or ethlyene cycle and two stages in the methane cycle. Consequently, it is obvious that original equipment costs could be reduced, spaced and weight requirements could be reduced, making installation on a barge or the like convenient, and the design and ease of estimating the system could be reduced if a single heat exchanger could be utilized in place of the conventional, multiple heat exchange units. It is also to be observed that each of the cooling stages of the propane and ethylene or ethane cycles is generally a tube and shell type heat exchanger in which the feed gas stream passes through the tubes while the refrigerant is expanded into the shell of the exchanger. The shell of the exchanger also functions as a phase separator to separate liquid phase refrigerant from flashed refrigerant so that the liquid phase refrigerant may be advanced to the next successive cooling stage. This, of course, requires larger than normal heat exchangers. It is also to be observed that in order to reduce the pressure of the feed gas stream to ambient pressure during the expansion cycle and, at the same time, utilize flashed vapors as the refrigerant in the methane cycle, it is necessary to provide a vapor-liquid separator for each expansion stage. It would, therefore, be highly desirable if these vapor liquid separators could be eliminated and less complex and cumbersome equipment could be utilized in place of the tube and shell type heat exchangers utilized conventionally.